Various fibrous materials, such as paper, paperboard, fiberboard and cellulose wet-lap, are made by forming a slurry or so-called stock of the fibrous material and other ingredients well dispersed in water and delivering the stock as a ribbon-like jet to a web former, such as the well-known Fourdrinier machine or a twin wire former. The mechanical properties and various other characteristics of the web produced by the web former are to a significant extent controlled by the characteristics of the stock delivered to the former from the headbox, and it is extremely important to the obtaining of high quality in the finished web that the fibers and other ingredients, such as fillers and additives, be thoroughly mixed in micro scale as well as macro scale. It is also important that the fibers be as free as possible of engagement with other fibers in the stock delivered from a headbox, i.e., that the fibers not be entangled in flocs.
Although to a considerable extent mixing of the stock occurs prior to feeding the stock to the headbox, one important function of a headbox is to give the stock a thorough final mixing so that the fibers and other ingredients are dispersed uniformly throughout the jet delivered through the slice opening. Among other important functions of a headbox are: the delivery of a precisely uniform thickness jet across the entire width of the former to ensure that the basis weight of the web is uniform; to deliver the stock jet at a controlled velocity to ensure uniformity of web properties from point to point lengthwise of the web and, particularly in twin wire formers, to control the mechanical properties of the web; to deliver a clean, smooth jet free of disturbances and irregularities, notably, machine direction streaks, waves due to velocity variations, water-hammer phenomena or wake effects from structures inside the headbox, and sprays of free drops.
Various headboxes that have been proposed or used over the past few decades have been equipped with elements intended to fulfill the mixing function of a headbox. Most of these elements are one or another form of obstruction interposed in the path of stock flow through the headbox to generate turbulence which, in turn, produces mixing. The obstructions have included baffles, perforated plates, rods, specially shaped vanes, perforated rolls and plates or sheets disposed parallel to the stock flow. Another approach has involved varying the cross section of the stock flow passages such as by providing projections extending from the walls of the headbox into the flow path. Vibrating plates and rods intended to induce mixing by mechanical vibration of the stock as it flows through the headbox have been tried. The various measures that have been proposed and used to enhance mixing of the stock in headboxes have, of course, been effective to various degrees, including some that have been quite successful. However, many of such measures have required trading off optimum results of one or more other functions of the headbox for improved mixing. For example, perforated plates are prone to clogging and to increasing the extent of floc formation due to the build up of flocs at the plates which break away periodically into the stock flow. Perforated plates and rods and vanes of various types also normally tend to produce wake effects that are reflected in streaks in the web. Some mixing devices have proven to be largely ineffective, for example, vibrating objects. Other mixing devices have been prone to mechanical failure from time to time, thus requiring shutdown of the machine for headbox repairs at very considerable costs in terms of lost production.